The present embodiments relate to a control apparatus for a current-regulated amplifier powering field coils of a magnetic resonance tomograph.
Known magnetic resonance tomography devices may include three gradient coils in order to generate linear magnetic field gradients in the x, y and z directions. A regulated current generated by a gradient or shim amplifier passes through each of the coils. The gradient amplifier includes a current controller for each gradient coil. The properties of the controller are adjusted for the connected gradient coil. A gradient amplifier of this type is described in German patent application DE 197 06 756 A1. The gradient amplifier operates at a purely inductive load, irrespective of the relatively low ohmic resistance losses. The current controller of the amplifier already controls the control deviation during a pulse slope to zero. An unwanted deformation of the pulse nevertheless occurs.
In order to avoid pulse deformations, patent application DE 198 56 800 C1 proposes adding a voltage to the controller output. The added voltage relieves the controller and is proportional to the gradient of the pulse slope of the gradient coil current. The output of an additional differentiation unit that is activated directly by the current desired value is present at the input of a final summation level by way of a calibration resistance. FIG. 1 shows a block diagram of a control apparatus of a gradient amplifier according to DE 198 56 800 C1. A current desired value I-SOLL is fed to a delay unit 21. An output of the delay unit 21 is connected to an input of a comparison unit 22. A current actual value I_IST is present at a further input of the comparison unit 22. A control differential signal R1, which is fed to a control unit 23, is determined in the comparison unit 22. The current desired value I_SOLL is also fed to a differentiation unit 24, which outputs a differential control signal D. The output signal of the control unit 23 and the differential control signal D are fed to inputs of a second summation unit 25. The control signal RS including the output signal of the control unit 23 and the differential control signal D is available at the output of the second summation unit.
In modern magnetic resonance tomography devices (e.g., Connectom by Siemens AG), each gradient coil includes a number of sub-coils. Each of the sub-coils is supplied with current by a gradient amplifier. This results in a magnetic coupling of the sub-coils. The magnetic coupling has repercussions on the gradient amplifier and influences the pulse shape of the gradients. For example, a strong coupling occurs between the Z sub-coils and the X and also Y sub-coils. This coupling is to be taken into account when designing the gradient amplifiers. In addition, the Z, X and Y sub-coils are also coupled to one another, which are essentially lower but still have a negative affect on the current course of the presently pulsing gradient amplifier and/or in the pulsed coil. The individual effect of each sub-coil on the remaining sub-coils (exception Z on X and Z on Y) is low, but the sum of the reactions of all sub-coils on the respectively pulsing sub-coil is not negligible.